NATURE OF POWER SATURATION IN TRAVELING WAVE TUBES 849 



everything else in Fig. 7, including subsequent variations of phase, are 

 measured. The solid line patterns represent the ac velocity, and the 

 shaded area, the charge density corresponding to that velocity. Thus in 

 each pattern we have a complete story of (fundamental) circuit voltage, 

 electron velocity and current density as a function of phase, for a par- 

 ticular signal input level. The velocity and current modulations at small 

 signal levels check calculated values well, and it is not difficult to visu- 

 alize the dynamics giving this pattern. 



Consider first the situation in the tube at small signal amplitudes. 

 At the input an unmodulated electron beam enters the field of an elec- 

 tromagnetic wave moving with approximately the same velocity as the 

 electrons. The electrons are accelerated or decelerated depending upon 

 their phase relative to the wave, and soon are modulated in velocity. 

 The velocity modulation causes a bunching of the electrons near the 

 potential maxima (i.e., the valleys in the inverted potential wave shown) 

 and these bunches in turn induce a new electromagnetic wave com- 

 ponent onto the circuit roughly in ciuadrature following the initial wave. 

 I'he addition of this component gives a net field somewhat retarded from 

 the initial wave and larger in amplitude. Continuation of this process 



Fig. 6 — Velocity analyzer patterns. The beam sample is made to traverse an 

 ellipse at }i the signal frequency. Current density modulation appears as intensity 

 variation, and velocity variation as vertical deflection from the ellipse. 



